I am trying to convince my boss our new database-server wants raid-0+1, not raid-5, and I got an idea while reading endless articles about raid-5 being slow when writing and management not listening.
suppose you make a 3-disc raid-5 without parity:
data disc1 disc2 disc3 A A A B B B C C
How would that perform compared to raid-5 and raid-0+1?
As I understand, the performance problem with raid-5 when writing is that you may need to read old data to recompute the parity block, and the write the parity block and the data in parallel.
So in this case, you can read straight away from all 3 disks (with raid-5 one of the disks will have parity information) and you can write without reading old data.
What you propose is not quite raid-5, but appears to be a striping hybrid between raid-0 and raid-1.
If I get what you are saying, you are writing each block twice, and ensuring each block is written to at least 2 different drives. Interesting idea, but not really efficient. In addition, you are doubling your write bandwidth requirements (not unlike pure raid-1).
Yes, raid-5 random write performance is limited by read-modify-write cycles when they are required. The raid-5 md driver does do a pretty good job of avoiding this where possible though.
I have not really benchmarked my raid-5 write performance, but on reads I sit around 90MB/s across my 10 drives. (Even my SATA raid-0 with 2 7200RPM drives has trouble getting much quicker than this as I keep saturating the PCI bus)
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